(1) Field of the Invention
The invention relates to an input-output optical system of spatially optical coupled type which is suitable for use in effecting optical coupling between an input and an output by sending and receiving light beam in a space without use of an optical fiber connection, as well as to an optical switch of spatially optical coupled type having the input-output optical system.
(2) Description of the Related Art
In association with recent speedup of an optical signal flowing through a trunk cable system, there arises a necessity for even an optical switch, such as an optical cross-connect device, to handle a high-speed optical signal having a speed exceeding 10 Gbps (gigabits per second). As a result of an increase in the number of wavelengths to be multiplexed (currently up to thousands of channels), a required scale of optical switching is becoming massive.
Against such a background, development of an optical switch of spatially optical coupled type using a micro tilt mirror array to which a micro electromechanical system (MEMS) is applied has recently been pursued as a technique for rendering the scale of an optical switch larger. There are available optical switches such as that described in, e.g., xe2x80x9cFully-provisioned 112xc3x97112 micro-mechanical optical cross-connect with 35.8T b/s demonstrated capacityxe2x80x9d by D. T. Neilson et al., Optical Fiber Communications Conference (OFC 2000), Post-deadline paper PD-12, March 2000,xe2x80x9d or that described in Published International Publication WO 00/20899. In relation to a micro tilt mirror using a MEMS, a technique proposed in, e.g., U.S. Pat. No. 6,044,705, has already been known.
In an optical switch of spatially optical coupled type using a mirror, an input-output optical system becomes important, wherein signal light having propagated through an (input) optical fiber is output into a space as a collimated beam, and the beam is subjected to processing, such as switching, through use of a micro tilt mirror. Then, the beam again enters an (output) optical fiber. For this reason, a technique for easily manufacturing a highly-accurate, stable input-output optical system is sought for commercializing a large-scale optical switch.
A conventional input-output optical system will now be described.
FIG. 17 schematically shows an example of three-dimensional mount structure of a conventional input-output optical system. The input-output optical system shown in FIG. 17 has a predetermined substrate 100; a pair of optical systems (optical transmission units) 200 mounted thereon with bolts or the like; i.e., one optical system for input and the other optical system for output (hereinafter, an input optical system 200 is sometimes denoted as an input optical system 200a, and an output optical system 200 is sometimes denoted as an output optical system 200b), each optical system being constituted by combination of a collimator lens 201 and a fiber block 202; an attachment member 300a on which the optical system 200a is to be mounted with bolts or the like; and an attachment member 300b on which the optical system 200b is to be mounted with bolts or the like. Here, illustration of an optical switch mechanism is omitted from FIG. 17. Here, the fiber block 202 is for housing a plurality of optical fibers in the form of an array.
In the input-output optical system having such a construction, in order to accurately align a light exist surface of the input optical system 200a to a light incidence surface of the output optical system 200b (i.e., to accurately align optical axes of the respective optical systems 200 with each other), an optical axis is aligned in a three-dimensional direction by means of individually adjusting mount positions and orientations (angles) of the optical systems 200 and those of the attachment members 300a, 300b. 
However, such a three-dimensional optical axis alignment requires alignment of six axes; that is, alignment of a longitudinal axis, alignment of a lateral axis, alignment of an optical axis, alignment of rotation around an optical axis, alignment of rotation around the longitudinal axis, and alignment of rotation around the lateral axis. Hence, an extremely large number of processes are required for assembling the input-output optical system. As described in, e.g., Japanese Patent Application Laid-Open No. 220405/1996, repetition of the following steps is required; namely, a step of fastening the attachment members 300a, 300b and the respective optical systems 200 with bolts, a step of detecting optical axes, and a step of temporarily loosening the bolts if no match exists between the optical axes and adjusting the mount positions and angles of the optical systems 200 and those of the attachment members 300a, 300b. Hence, adjustment of optical axes requires consumption of much time. For this reason, an improvement in manufacturing yield is not expected, resulting in high product costs.
Proposed in the aforesaid patent publication 220405/1996 is an optical transmission unit 2 such as that shown in FIGS. 18A and 18B. FIG. 18A is a front view of the optical transmission unit 2, and FIG. 18B is a transverse plan view of the optical transmission unit 2.
In the optical transmission unit 2, the amount of fastening of respective fastening screws 19 is adjusted individually through use of the three mount screws 19 and three springs (compression coil springs) 20, thereby adjusting the angle of an outgoing beam (i.e., a beam into which the light output from a light-emitting element 5 is collimated by a collimator lens 3).
In FIGS. 18A and 18B, reference numeral 4 designates a polarization beam splitter; 5 designates a light-emitting element; 6 designates a light-receiving element; 7 designates an optical axis; 11 designates a main body frame; 9 designates a front frame of the main body frame 11; 9a designates an attachment hole formed in the front frame 9; 9b designates screw holes; 14 designates a cylindrical section inserted into the attachment hole 9a so that the angle of the cylindrical section can be displaced; 16 designates a flange; 15 designates an attachment hole by which it is formed in the flange 16 perimeter at intervals of 120 degrees, and the screw 19 is inserted; 17 designates a cylinder; 18 designates an element unit formed by attaching the polarization beam splitter 4, the light-emitting element 5, and the light-receiving element 6 to the cylinder 17; and 19a designates the head of a mount screw 19.
Under the foregoing known technique, adjustment of an optical axis becomes easier than that shown in FIG. 17. However, use of the springs 20 results in unstable fastening of the unit, which may cause an unexpected offset in an optical axis. Further, an offset may arise in an optical axis for reasons of long-term variations in elastic moduli of the springs 20. Therefore, the known technique is insufficient in terms of accuracy and reliability (stability) of adjustment of an optical axis.
The invention has been conceived in light of such a drawback and aims at providing an input-output optical system of spatially optical coupled type and an optical switch, which enable highly accurate, highly stable, and easy alignment of an optical axis.
To achieve the object, the invention provides an input-output optical system of spatially optical coupled type comprising:
a substrate;
an input optical system which is provided on the substrate and which has an input fiber block and an input lens array block, wherein a plurality of input optical fibers are connected to the input fiber block in an array, and a plurality of collimating lenses which collimate light input from the optical fibers connected to the input fiber block and output collimated light are arranged in the input lens array block in an array;
an output optical system which is provided on the substrate and which has an output lens array block and an output fiber block, wherein a plurality of collimating lenses which collimate respective light rays output from the input lens array block are arranged in the output lens array block in an array, and an output fiber block to which a plurality of output optical fibers are connected in an array and which output the light output from the output lens array block to the output optical fibers; and
a spacer which is interposed between the lens array block and the fiber block in at least one of the input and output optical systems without blocking an optical path for ensuring a distance corresponding to a focal distance of the collimating lenses and a distance of the optical path.
By means of such a configuration, in the input-output optical system of the invention, a distance corresponding to a focal distance of the collimating lenses is provided between the lens array block and the fiber block without posing hindrance to an optical path, by means of the spacer. Hence, alignment of the input-output optical system in an axial direction becomes unnecessary, and hence alignment of an optical axis is achieved through a two-dimensional positional adjustment within a single plane.
Therefore, alignment of an optical axis which is more accurate and stable than conventional alignment of an optical axis can be easily implemented.
Here, the spacer is preferably formed from a plate-like transparent member which has a thickness corresponding to the focal distance and the distance of the optical path in a direction of the optical path and allows transmission of the light. In this case, the plate-like transparent member preferably has an optical refractive index corresponding to that of the input optical fibers or that of the output optical fibers, and the lens array block and the fiber block are preferably cemented together with an adhesive having the same refractive index as that of the optical index. As a result, the optical path is not hindered, and the amount of light reflected from a cemented portion can be reduced.
Further, the plate-like transparent member maybe formed by combination of a plurality of transparent plates having wedge-shaped side surfaces such that a thickness in the direction of an optical path is changed as a result of sliding of the transparent plates. By means of such a configuration, alignment of an optical path (i.e., adjustment of the distance of an optical path) can be effected readily and with high precision.
Accordingly, the distance between the optical fibers and the collimating lenses can be adjusted readily and with high precision.
The spacer may be formed from a plate-like member which has a thickness corresponding to the focal distance and the distance of the optical path in a direction of the optical path, so as to avoid hindering the optical path in accordance with the arrangement of the collimating lenses. Even in the case of such a configuration, alignment of an optical axis (i.e., adjustment of distance of an optical path) becomes unnecessary, and hence the only requirement is to perform two-dimensional alignment of an optical axis within a single plane.
Moreover, the space may be embodied as any space, so long as it does not hinder an optical path. For instance, the space may be embodied as a plate-like transparent member which transmits light or is formed into a shape so as to avoid hindering an optical path. When the transparent path is employed, the spacer preferably has an optical refractive index corresponding to the optical refractive index of the optical fibers. The lens array block and the fiber block are preferably cemented together with an adhesive having the same refractive index as the optical refractive index. As a result, an optical loss or reflection which would arise in cemented surfaces can be reduced.
The substrate and the fiber block may be provided with positioning means for fixing the fiber block at a predetermined position on the substrate.
As a result, the fiber block can be fixed at a predetermined position after having been positioned readily and with high accuracy.
Further, a return mirror may further be provided on the substrate for shifting light output from the input optical system to a predetermined direction, thus reflecting the light. In this case, the input optical system and the output optical system can be assembled into a single input-output-integrated block such that a surface of the input optical system by way of which the light is output and a surface of the output optical system by way of which the light reflected by the return mirror enters are located within a single plane. As a result, there is obviated a necessity for individually positioning the input and output optical systems.
Accordingly, easier, highly accurate mounting of input and output optical systems becomes feasible.
Here, the return mirror may be fittingly fixed at a predetermined position on the substrate. This also enables easy positioning (fixing) of the return mirror, and hence easier mounting of the input-output optical system becomes feasible.
Further, the input-output-integrated block may be provided with a positioning member for temporarily positioning the return mirror and the input-output-integrated block on the substrate, and positioning means to be fixed at a predetermined position. By means of such a configuration, there is obviated a necessity for individually positioning the return mirror and the input-output-integrated block.
Accordingly, the return mirror and the input-output-integrated block can be positioned and fixed more easily with higher accuracy.
If the fiber block and the lens array block are formed from materials having equal coefficient of thermal expansion, there is suppressed occurrence of an offset in relative positions which would be caused by a temperature difference between the fiber block and the lens array block attributable to a difference in coefficient of thermal expansion. A positional relationship between the centers of the optical fibers and the centers of the lenses can be made constantly regardless of temperature, thereby enabling a reduction in temperature variations of the angle of the light output from the input optical system. Hence, an input optical system which is very stable against thermal variations can be realized.
Moreover, the fiber block is constituted of a metal insert member and resin material. A plurality of holes corresponding to the arrangement of optical fibers are formed in the metal insert member, and the metal insert member has the same coefficient of thermal expansion as that of the lens array block. The resin material covers the insert member. In this case, the fiber block having a coefficient of thermal expansion equal to that of the lens array block can be readily prepared.
A spatial optical switch of the invention is characterized by comprising the foregoing input and output optical systems and a tilt mirror array block for effecting switching of an optical path existing between the input optical system and the output optical system. As a result, even in the spatial optical switch of the invention, a distance corresponding to the focal distance of the collimating lens is assured between the lens array block and the fiber block without posing a hindrance to an optical path by means of the spacer. Hence, alignment of the optical switch in an axial direction (i.e., adjustment of distance of an optical path) becomes unnecessary, and hence the only requirement is two-dimensional adjustment of an optical axis.
Therefore, an optical switch of spatially optical coupled type which is easy to assemble and enables easy and highly accurate alignment of an optical axis can be realized, thereby enabling an attempt to reduce the cost of such an optical switch.